Electronic structures and spectroscopic properties of [Pt(CNMe) ₂(CN)₂]_n (n = 1-4): A theoretical exploration of promising phosphorescent materials

The structures of [Pt(CNMe)₂(CN)₂]_n (n = 1-4) in the ground states (S₀) and lowest-energy triplet excited states (T₁) were calculated by using the second-order Møller-Plesset perturbation (MP2) and density functional theory (DFT) methods, respectively. The MP2 results show that the formation of the dimer causes a significant red shift in emission energy, and the frequency calculations reveal that a weak metal-metal interaction exists in the S ₀ state, which is greatly enhanced in the ³[d _σ·p_σ] excited state. The aggregation of [Pt(CNMe)₂-(CN)₂]_n (n = 1-4) was explored by using the slate-type VWN functional in the DFT method. The ³B _u→¹A_g transition in the dimer at 509 nm corresponds to the experimental higher-energy emission at 530 nm in CH ₃CN solution, while the ³A′→ ¹A′ transitions in the trimer and tetramer at 557 and 650 nm, respectively, are responsible for the low-energy emission at 584 nm observed experimentally. The analyses of the Wiberg bond indices for the Pt-Pt bond indicate that the dimer may be the most stable form in solution and that the oligomer species (n = 3 and 4) can be treated as a special dimer in which the excess z electron ligand is bonded to the Pt atoms of the central dimer.